This invention relates to a method of coating a yarn and, more particularly, to a method of coating a fiberglass yarn with a hot melt thermoplastic composition by applying the thermoplastic as a melt to the yarn from a heated bath, then drawing the thus coated yarn through a heated die to force the hot melt composition into the central areas of the yarn and leave a uniform, smooth coating on the outside of the yarn, and finally cooling the coated yarn at ambient temperatures to allow the melt to return to its solid state.
Glass yarns coated with hot melt compositions have several uses. One use is to bind the edges of a glass fabric woven on a shuttleless loom, as described in copending, commonly assigned application Ser. No. 863,725 filed May 16, 1986, entitled "Bonded Glass Fabric Edge" to prevent the edges from raveling. There are various procedures for applying hot melt compositions to the glass yarn. One procedure is to apply a uniform coating of a hot melt thermoplastic composition to a fiberglass yarn with thorough impregnation of the thermoplastic into the yarn, such as kiss coating the yarn. However, kiss coating has certain disadvantages including low loading and much variation. In the kiss coat method a grooved roll is partially immersed in a bath of a heated hot melt composition, and prior to contact with the yarn, excess melt is wiped from the roll. This is followed by directing a passing yarn into the melt-filled v's of the grooved roll. The product that resulted was uneven with beaded areas along the length of the yarn and often had zones of excessive amounts of melt adhered to certain areas of the yarn. When high levels of coating are to be applied to a yarn, for instance coating levels approximating the weight of the uncoated yarn itself, expressed as 100% pickup, kiss coating with a grooved roll, as described, results in an uneven, beaded coating. In addition to the type of coating procedure employed, variations in temperature, melt viscosity, roll speed, and yarn speed also affected the resin pick up and thus consistency of the coated yarn.
Dies have been used for several years to coat yarns. These die coating procedures apply a liquid coating solution, i.e. an aqueous solution, solvent solution, plastisol, organisol, etc., to the yarn. The yarn is then passed through a coating die of the prescribed orifice size which squeezes the coating into the yarn bundle and doctors off excess coating. The yarn is next passed through an oven to dry by evaporating the solvent and/or curing the coating depending upon the nature of the coating composition applied. The drying procedure requires considerable investment in drying equipment and additional time to fully dry/cure the yarn prior to windup on a reel.
A distinction must be made between the use of liquid polymers or polymer solutions and hot melt coatings. The liquid polymers or solutions of polymers dissolved in a solvent, typically an epoxy resin dissolved in a volatile organic solvent, require the use of heat to drive off the solvent and/or cure the resin that is coated on the yarn. In contrast to the resin solution approach, in which the resin once cured cannot be returned to a mobile state, this invention features the use of hot melts which are applied in the molten state and solidify on cooling. This solidification is reversible and the coating can be made molten once again with heat. This reversibility is an important feature when the hot melt coated yarn is used as a leno edge bonding for woven glass cloth.